A belt unit includes an endless transfer belt, a driving roller that rotationally drives the belt, and a driven roller that is rotationally driven with the rotation of the transfer belt looped between the driving roller and the driven roller.
However, if a tension applied to the belt is not uniform in a width direction or if deviations in dimensions or assembly dimensions of the respective rollers are large, the belt skews, so that the belt moves in the width direction while being rotated. Herein, the width direction means a direction that is orthogonal to a rotation direction and is orthogonal to a thickness direction of the belt, and corresponds to an axial direction of the roller in general.
Further, if the belt largely skews, the end portion of the belt in the width direction interferes with other members, so that it is concerted that the end portion of the belt in the width direction is damaged.
Accordingly, for example, in a related-art technique, the skew of the belt is suppressed by displacing the rotating shaft of the driven roller in an upper-lower direction, which is a direction orthogonal to an extension surface of the belt, according to the skew of the belt. Herein, the extension surface means a portion of the belt, which is extended in a flat surface shape by the application of a tension. In general, the extension surface means a flat surface portion of the belt that is formed between the driving roller and the driven roller.